This application claims priority under 35 USC §119 to Korean Patent Application No. 2007-0045516, filed on May 10, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to pulse width modulation (PWM) signal generators, and more particularly to a PWM signal generator that generates an asymmetric PWM signal and a data processing apparatus including the same.
2. Background of the Invention
Pulse width modulation (PWM) is a modulation technique that adjusts the width of pulses according to the level of an input signal. For high efficiency, high resolution, and low power consumption, PWM is widely used in amplifiers (e.g., Class-D amplifiers) or in a data processing apparatus (e.g., audio processing apparatus).
A PWM input signal is a quantized signal. To convert a signal through quantization, delta sigma modulation (DSM) which reduces quantization noise with high resolution is widely used. Especially, in the field of audio amplifiers for driving speakers or headphones requiring high efficiency, Class-D amplification using DSM and PWM is usually used.
DSM is a technology for obtaining high resolution using noise shaping and over-sampling. Noise shaping frequency-shifts quantization noise generated in a signal band during signal quantization into a no-signal band (i.e., a band that is not used). The amount of noise frequency-shifted into the no-signal band is proportional to a loop filter order of a modulator.
Over-sampling is a process of sampling a signal with a frequency significantly higher than the bandwidth of the signal. Since the frequency band is extended by the over-sampling, the level of quantization noise is decreased. Here, as the over-sampling frequency, i.e., an over-sampling ratio (OSR) is increased, the level of the quantization noise is decreased, so that a signal-to-noise-and-distortion ratio (SNDR) in the bandwidth is increased. Accordingly, the SNDR may be increased in the bandwidth of a signal by using the over-sampling and the noise shaping in a DSM device.
A quantized multi-bit signal generated from the DSM device is converted into a pulse width modulation (PWM) signal by a pulse width modulator. Usually, a PWM signal is a digital pulse signal symmetric with respect to the center of a pulse period. In other words, a PWM signal has pulse widths symmetric with respect to such a center of each pulse period. With such a symmetric PWM signal, a linear characteristic is ensured when the PWM signal is converted into an analog signal by a low-pass filter (e.g., an LC filter). In addition with such a symmetric PWM signal, total harmonic distortion (THD) is reduced and a signal-to-noise ratio (SNR) in increased.
For maintaining the symmetry of a PWM signal, the operating frequency of a delta sigma modulator is desired to be same as the central frequency of a PWM signal. Consequently, when the central frequency of a PWM signal is defined, the operating frequency of a DSM device is determined such that the over-sampling ratio and in turn the SNR are restricted.